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Orbital-optimized opposite-spin scaled second order correlation: An economical method to improve the description of open-shell molecules

Description: Coupled cluster methods based on Brueckner orbitals are well-known to resolve the problems of symmetry-breaking and spin-contamination that are often associated with Hartree-Fock orbitals. However their computational cost is large enough to prevent application to large molecules. Here they present a simple approximation where the orbitals are optimized with the mean-field energy plus a correlation energy taken as the opposite-spin component of the second order many-body correlation energy, scaled by an empirically chosen parameter (recommended as 1.2 for general applications). This optimized 2nd order opposite spin (abbreviated as O2) method requires fourth order computation on each orbital iteration. O2 is shown to yield predictions of structure and frequencies for closed shell molecules that are very similar to scaled second order Moller-Plesset methods. However it yields substantial improvements for open shell molecules, where problems with spin-contamination and symmetry breaking are shown to be greatly reduced.
Date: January 1, 2007
Creator: Lochan, Rohini C. & Head-Gordon, Martin
Partner: UNT Libraries Government Documents Department

Atomic electron correlations in intense laser fields

Description: Abstract. This talk examines two distinct cases in strong opbical fields where electron correlation plays an important role in the dynamic.s. In the first. example, strong coupling in a two-electron-like system is manifested as an intensity-dependent splitting in the ionized electron energy distribution. This two-electron phenomenon (dubbed continuum-continuum Autler-Townes effect) is analogous to a strongly coupled two- level, one-electron atom but raises some intriguing questions regarding the exact nature of electron-electron correlation. The second case examines the evidence for two-electron ionization in the strong-field tunneling limit. Although our ability to describe the one- electron dynamics has obtained a quantitative level of understanding, a description of the two (multiple) electron ionization remains unc
Date: September 3, 1998
Creator: Agostini, P. A.; DiMauro, L. F.; Kulander, K.; Sheehy, B. & Walker, B.
Partner: UNT Libraries Government Documents Department

Continuum-continuum Autler-Townes splitting in calcium

Description: Strong-field ionization of two-electron atoms can result in scenarios in which the electron-electron correlation plays an important role. It was recently suggested that a splitting similar to the Autler-Townes effect would occur when two ionization continua are resonantly coupled in two-electron atoms. This is obviously at variance with the case of one electron atoms where coupling between continua does not induce oscillations but instead leads to exponential decay of one continuum into another. The special case considered by Grobe and Eberly is that of a strong radiation field resonantly coupling two ionic states. Formally, the states which are coupled are continuum states (two-electron states in which one electron is in a continuum state), but nevertheless the corresponding photoelectron peak is split. Physically, the reason for this is that the electron-electron interaction transfers the energy shift of the core electron to the outgoing electron and has been dubbed {open_quotes}coherence transfer{close_quotes} by Ref.
Date: October 1, 1995
Creator: Walker, B.; Sheehy, B. & Kaluza, M.
Partner: UNT Libraries Government Documents Department

Structure and Dynamics of Negative Ions

Description: This report describes progress made during the final three-year grant period 1997-2000. During this period, we experimentally investigated the structure and dynamics of negative ions by detaching the outermost electron in controlled processes induced by photon-, electron- and heavy particle-impact. In this manner we studied, at a fundamental level, the role of electron correlation in the structure and dynamics of simple, few-particle atomic systems. Our measurements have provided sensitive tests of the ability of theory to go beyond the independent electron model.
Date: October 24, 2000
Partner: UNT Libraries Government Documents Department

Observation of Dynamical spin shielding in Ce: Why It Matters for Pu Electronic Structure

Description: In a series of experiments and linked theoretical modeling, the range of possible solutions for Pu electronic structure has been dramatically reduced. Nevertheless, the key issue of electron correlation remains.
Date: March 8, 2007
Creator: Tobin, J G; Yu, S W; Chung, B W; Morton, S A; Komesua, T & Waddill, G D
Partner: UNT Libraries Government Documents Department

An Alternative Model for Electron Correlation in Pu

Description: Using a density functional theory based approach that treats the 5f electrons relativistically, a Pu electronic structure with zero net magnetic moment is obtained, where the 5f orbital and 5f spin moments cancel each other. By combining the spin and orbital specific densities of states with state, spin and polarization specific transition moments, it is possible to reconstruct the experimentally observed photoemission spectra from Pu. Extrapolating to a spin-resolving Fano configuration, it is shown how this would resolve the extant controversy over Pu electronic structure.
Date: October 23, 2007
Creator: Yu, S; Tobin, J & Soderlind, P
Partner: UNT Libraries Government Documents Department

Facilities for the Performance of Fano Effect Measurements as a Probe of Electron Correlation

Description: Fano Effect measurements are the key to direct observation of the Kondo or spin shielding intrinsic to models of electron correlation. The Fano Effect is the observation of spin polarized photoelectron emission from NONMAGNETIC materials, under chirally selective excitation, such as circularly polarized photons. Below are described three spectrometers, with which Fano Effects measurements have been made.
Date: November 8, 2006
Creator: Tobin, J G; Yu, S W; Komesu, T; Chung, B W; Morton, S A & Waddill, G D
Partner: UNT Libraries Government Documents Department

The Utilization of Spin Polarized Photoelectron Spectroscopy as a Probe of Electron Correlation with an Ultimate Goal of Pu

Description: We are developing the technique of spin-polarized photoelectron spectroscopy as a probe of electron correlation with the ultimate goal of resolving the Pu electronic structure controversy. Over the last several years, we have demonstrated the utility of spin polarized photoelectron spectroscopy for determining the fine details of the electronic structure in complex systems such as those shown in this report.
Date: February 7, 2008
Creator: Tobin, J G; Yu, S W; Chung, B W; Morton, S A; Komesu, T & Waddill, G D
Partner: UNT Libraries Government Documents Department

Correlation effects in the iron pnictides

Description: One of the central questions about the iron pnictides concerns the extent to which their electrons are strongly correlated. Here we address this issue through the phenomenology of the charge transport and dynamics, single-electron excitation spectrum, and magnetic ordering and dynamics. We outline the evidence that the parent compounds, while metallic, have electron interactions that are sufficiently strong to produce incipient Mott physics. In other words, in terms of the strength of electron correlations compared to the kinetic energy, the iron pnictides are closer to intermediately-coupled systems lying at the boundary between itinerancy and localization, such as V{sub 2}O{sub 3} a or Se-doped NiS{sub 2} , rather than to simple antiferromagnetic metals like Cr. This level of electronic correlations produces a new small parameter for controlled theoretical analyses, namely the fraction of the single-electron spectral weight that lies in the coherent part. Using this expansion parameter, we construct the effective low-energy Hamiltonian and discuss its implications for the magnetic order and magnetic quantum criticality. Finally, this approach sharpens the notion of magnetic frustration for such a metallic system, and brings about a multi band matrix t-J{sub 1}-J{sub 2} model for the carrier-doped iron pnictides.
Date: January 1, 2009
Creator: Zhu, Jian-xin; Si, Qimiao; Abrahams, Elihu & Dai, Jianhui
Partner: UNT Libraries Government Documents Department

Conference summary: Experimnetal

Description: The conference is the 1995 International Conference on Strongly Correlated Electron Systems. The summary highlights research on the ``extended`` Doniach model, Kondo insulators, borocarbide superconductors, oxides (including cuprates), other phase transitions, and new materials.
Date: December 31, 1995
Creator: Thommpson, J.D.
Partner: UNT Libraries Government Documents Department

Strongly correlated electronic materials

Description: This is the final report of a 3-year project. Novel electronic materials characterized by strong electronic correlations display a number of unexpected, often extraordinary, properties. These are likely to play a major role in purpose-specific high-technology electronic materials of the future developed for electronic, magnetic, and optical applications. This project sought to develop predictive control of the novel properties by formulating, solving and applying many-body models for the underlying microscopic physics. This predictive control required the development of new analytical and numerical many-body techniques and strategies for materials of varying strengths of interactions, dimensionality and geometry. Results are compared with experiment on classes of novel materials, and the robust techniques are used to predict additional properties and motivate key additional experiments.
Date: April 1, 1996
Creator: Bedell, K.; Albers, R.; Balatsky, A.; Bishop, A.; Bonca, J.; Gubernatis, J. et al.
Partner: UNT Libraries Government Documents Department

A computational study of ethane cracking in cluster models of zeolite H-ZSM-5.

Description: Protolytic cracking of ethane by zeolites has been studied using quantum-chemical techniques and a cluster model of the zeolite Broensted acid site. Previous computational studies have utilized small cluster models and have not accounted for the long-range effects of the zeolite lattice. These studies have found reaction barriers for cracking which are significantly higher than experimental values. In this work we used a larger zeolite cluster model containing five tetrahedral (Si, Al) atoms (denoted 5T) and searched for stationary points along one possible reaction path for cracking at the HF/6-31 G(d) level of theory. This path involves a multi-step cracking reaction, in which the proton is first transferred from the acid site to the adsorbed ethane molecule to form an ion-pair equilibrium complex. Subsequently the proton attacks the C-C bond to complete the cracking process. The activation barrier for cracking was calculated, including corrections for (i) vibrational energies at the experimental reaction temperature of 773 K; (ii) electron correlation and an extended basis set at the B3LYP/6-311+G(3df,2p) level; and (iii) the influence of the surrounding zeolite lattice in H-ZSM-5. The barrier we obtain, 53 {+-} 5 kcal/mol, is significantly smaller than previous theoretical results and is in good agreement with typical experimental values for small hydrocarbons. Work is currently in progress to extend this study by carrying out geometry optimization of these complexes using the B3LYP method of density functional theory.
Date: August 21, 1998
Creator: Zygmunt, S. A.
Partner: UNT Libraries Government Documents Department

Nucleation of nanocrystalline diamond by fragmentation of fullerene precursors.

Description: Growth of diamond films from C{sub 60}/Ar microwave discharges results in a nanocrystalline microstructure with crystallite sizes in the range 3-10 nm. Heterogeneous nucleation rates of 10{sup 10} cm{sup {minus}2} sec are required to account for the results. The nucleation mechanism presented here fulfills this requirement and is based on the insertion of carbon dimer, C{sub 2}, molecules, produced by fragmentation of C{sub 60}, into the n-bonded dimer rows of the reconstructed (100) surface of diamond. Density functional theory is used to calculate the energetic of C{sub 2} insertion into carbon clusters that model the (100) surface. The reaction of singlet C{sub 2} with the double bond of the C{sub 9}H{sub 12} cluster leads to either carbene structures or a cyclobutynelike structure. At the HF/6-31G* level, the carbene product has a C{sub 2v} structure, while at the B3LYP/6-31G* levels of theory, it has a C{sub s} structure with the inserted C{sub 2} tilted. No barrier for insertion into the C=C double bond of the C{sub 9}H{sub 12} cluster was found at the HF/6-31G* and B3LYP/6-31G* levels of theory. Thus, calculations including correlation energy and geometry optimization indicate that insertion of C{sub 2} into a C=C double bond leads to a large energy lowering, {approximately}120 kcal/mol for a C{sub 9}H{sub 12} cluster, and there is no barrier for insertion.
Date: May 4, 1998
Creator: Gruen, D. M.
Partner: UNT Libraries Government Documents Department

Scaled Opposite Spin Second Order Moller-Plesset Correlation Energy: An Economical Electronic Structure Method

Description: A simplified approach to treating the electron correlation energy is suggested in which only the alpha-beta component of the second order Moller-Plesset energy is evaluated, and then scaled by an empirical factor which is suggested to be 1.3. This scaled opposite spin second order energy (SOS-MP2) yields results for relative energies and derivative properties that are statistically improved over the conventional MP2 method. Furthermore, the SOS-MP2 energy can be evaluated without the 5th order computational steps associated with MP2 theory, even without exploiting any spatial locality. A 4th order algorithm is given for evaluating the opposite spin MP2 energy using auxiliary basis expansions, and a Laplace approach, and timing comparisons are given.
Date: August 2, 2004
Creator: Jung, Yousung; Lochan, Rohini C.; Dutoi, Anthony D. & Head-Gordon, Martin
Partner: UNT Libraries Government Documents Department

Correlation effects on stability in Pu metal and its alloys

Description: The existence of six crystallographic allotropes from room temperature up to the solid-liquid transition just above 913 K at atmospheric pressure makes solid Plutonium unique among the elements in the periodic table. Among these phases (labeled {alpha}, {beta}, {gamma}, {delta}{delta}{prime}), and {var_epsilon}, the {delta} phase, stable between 593 K and 736 K, has commanded considerable interest in the metallurgical and solid state communities. In contrast to the low-temperature monoclinic {alpha} phase, which is brittle, the face-centered cubic (fcc) {delta} phase is ductile, a property that makes it convenient for engineering applications. This phase can also be stabilized through alloying with a number of other elements such as Ga, Al, Sc, and Am.
Date: April 1, 1999
Creator: Cooper, B R; Gonis, A; Kiousis, N; Price, D L & Turchi, P E
Partner: UNT Libraries Government Documents Department

Core-Level Satellites and Outer Core-Level Multiplet Splitting in Mn Model Compounds

Description: We report a systematic study of the Mn 2p, 3s and 3p core-level photoemission and satellite structures for Mn model compounds. Charge-transfer from the ligand state to the 3d metal state is observed and is distinguished by prominent shake-up satellites. We also observe that the Mn 3s multiplet splitting becomes smaller as the Mn oxidation state increases, and that 3s-3d electron correlation reduces the branching ratio of the 7S:5S states in the Mn 3s spectra. In addition, as the ligand electronegativity decreases, the spin state purity is lost in the 3s spectra as evidenced by peak broadening. Our results are best understood in terms of the configuration-interaction (CI) model including intrashell electron correlation, charge-transfer and final-state screening.
Date: October 18, 1999
Creator: Nelson, A.J.; Reynolds, J.G. & Roos, J.W.
Partner: UNT Libraries Government Documents Department

Theory of exotic superconductivity and normal states of heavy electron and high temperature superconductivity materials. Progress report, February 15, 1994--February 14, 1995

Description: This is a progress report for the DOE project covering the period 2/15/94 to 2/14/95. The PI had a fruitful sabbatical during this period, and had some important new results, particularly in the area of new phenomenology for heavy fermion superconductivity. Significant new research accomplishments are in the area of odd-in-time-reversal pairing states/staggered superconductivity, the two-channel Kondo lattice, and a general model for Ce impurities which admits one-, two-, and three-channel Kondo effects. Papers submitted touch on these areas: staggered superconductivity - a new phenomenology for UPt{sub 3}; theory of the two-channel Kondo lattice in infinite dimensions; general model of a Ce{sup 3+} impurity. Other work was done in the areas: Knight shift in heavy fermion alloys and compounds; symmetry analysis of singular pairing correlations for the two-channel Kondo impurity model.
Date: February 1, 1995
Creator: Cox, D. L.
Partner: UNT Libraries Government Documents Department

Excitations, optical absorption spectra, and optical excitonic gaps of heterofullerenes: I. C60, C59N+ and C48N12

Description: Low-energy excitations and optical absorption spectrum of C{sub 60} are computed by using time-dependent (TD) Hartree-Fock (HF), TD-density functional theory (TD-DFT), TD-DFT-based tight-binding (TD-DFT-TB) and a semiempirical ZINDO method. A detailed comparison of experiment and theory for the excitation energies, optical gap and absorption spectrum of C{sub 60} is presented. It is found that electron correlations and collective effects of exciton pairs play important roles in assigning accurately the spectral features of C{sub 60} and the TD-DFT method with non-hybrid functionals or a local spin density approximation leads to more accurate excitation energies than with hybrid functionals. The level of agreement between theory and experiment for C{sub 60} justifies similar calculations of the excitations and optical absorption spectrum of a monomeric azafullerene cation C{sub 59}N{sup +} exhibits distinguishing spectral features different from C{sub 60}: (1) the first singlet is dipole-allowed and the optical gap is redshifted by 1.44 eV; (2) several weaker absorption maxima occur in the visible region; (3) the transient triplet-triplet absorption at 1.60 eV (775 nm) is much broader and the decay of the triplet state is much faster. The calculated spectra of C{sub 59}N{sup +} characterize and explain well our measured ultraviolet-visible (UV-vis) and transient absorption spectra of the carborane anion salt [C{sub 59}N][Ag(CB{sub 11}H{sub 6}Cl{sub 6}){sub 2}]. For the most stable isomer of C{sub 48}N{sub 12}, we predict that the first singlet is dipole-allowed, the optical gap is redshifted by 1.22 eV relative to that of C{sub 60}, and optical absorption maxima occur at 585, 528, 443, 363, 340, 314 and 303 nm. We point out that the characterization of the UV-vis and transient absorption spectra of C{sub 48}N{sub 12} isomers is helpful in distinguishing the isomer structures required for applications in molecular electronics. For C{sub 59}N{sup +} and C{sub 48}N{sub 12} as well as ...
Date: October 2, 2003
Creator: Xie, R; Bryant, G W; Sun, G; C. Nicklaus, M; Heringer, D; Frauenheim, T et al.
Partner: UNT Libraries Government Documents Department

Theoretical aspects of electron correlations in electron collisions

Description: Electron-correlation effects manifest themselves in diverse facets of electron collisions with an atom or molecule. A fast incident electron acts largely as an external agent, and its inelastic collisions probe the structure, including correlations within the target (in both the initial state and the final state), nearly in the same way as photoionization processes do. In inelastic collisions of a slow electron, the central object of study is the correlated motion of the incident electron and an electron excited out of the target core. This elementary observation is illustrated in the lecture by many examples and is elaborated by remarks on some current theoretical methods. (auth)
Date: January 1, 1975
Creator: Inokuti, M.
Partner: UNT Libraries Government Documents Department

Experimental Bench-marking of Pu Electronic Structure

Description: Our plan is to do Ce (as a Pu surrogate) this year and be ready to do Pu next year. The Fano (Spin-resolved Photoelectron Spectroscopy) measurements are essential to testing electron correlation in the occupied 5f states. BIS (Bremstrahlung Isochromat Spectroscopy or high energy Inverse Photoelectron Spectroscopy) experiments are crucial to a quantitative determination of the 5f unoccupied density of states (5f-UDOS). The 5f UDOS is the key to differentiation between a myriad of models of 5f electronic structure. During this time, we will work to converge to a solution for the Pu safety issues, with the plan to implement these in the next FY. Acceleration of this schedule and implementation of the safety plan in this FY will require a very significant increase in funding. Ultimately, results from the Pu experiments will be fed into calculations performed by P. Soderlind, A. Landa, and others.
Date: July 31, 2007
Creator: Laboratory, Lawrence Livermore National
Partner: UNT Libraries Government Documents Department

Experimental Benchmarking of Pu Electronic Structure

Description: The standard method to determine the band structure of a condensed phase material is to (1) obtain a single crystal with a well defined surface and (2) map the bands with angle resolved photoelectron spectroscopy (occupied or valence bands) and inverse photoelectron spectroscopy (unoccupied or conduction bands). Unfortunately, in the case of Pu, the single crystals of Pu are either nonexistent, very small and/or having poorly defined surfaces. Furthermore, effects such as electron correlation and a large spin-orbit splitting in the 5f states have further complicated the situation. Thus, we have embarked upon the utilization of unorthodox electron spectroscopies, to circumvent the problems caused by the absence of large single crystals of Pu with well-defined surfaces. Our approach includes the techniques of resonant photoelectron spectroscopy [1], x-ray absorption spectroscopy [1,2,3,4], electron energy loss spectroscopy [2,3,4], Fano Effect measurements [5], and Bremstrahlung Isochromat Spectroscopy [6], including the utilization of micro-focused beams to probe single-crystallite regions of polycrystalline Pu samples. [2,3,6]
Date: October 13, 2005
Creator: Tobin, J G; Moore, K T; Chung, B W; Wall, M A; Schwartz, A J; Ebbinghaus, B B et al.
Partner: UNT Libraries Government Documents Department